The effects of placing non-magnetic, electrically conductive objects in the alternating magnetic fields of coils have been the subject of much study and are generally viewed as predictable and understood. In this vein, the magnetic coupling which provides the basis for mutual inductance has long been employed to achieve energy transfer, impedance matching and loading. In addition, these same effects provide the basis of development for more esoteric devices such as proximity sensors, contactless thickness measuring devices, position detecting systems, accelerometers, electronic tachometers and monitoring devices of various kinds.
Typically, when a non-magnetic, electrically conductive object is placed in the alternating magnetic field of a coil, the effective inductance of the coil is decreased and its effective resistance is increased. This occurs because whenever the magnetic flux in a medium is changing, an electric field appears within the medium as a result of time variation of the flux. When the medium is conductive, circulating currents tending to oppose the field or the externally applied magnetomotive force are created. These currents are called eddy currents and their presence results in an energy loss in the material proportional to the energy being absorbed from the circuit that sets up the field and being dissipated as heat in the medium. The effect of such currents is to screen or shield the material from the flux and to bring about a smaller flux density nearer the center of the object than at the surface. Thus, for a specific total flux varying periodically, the maximum flux density at the center is smaller than what would be obtained from dividing the total maximum flux by the area. In effect, the total flux tends to be crowded toward the surface of the object to create an effect known as skin effect, which is quite similar to the skin effect phenomena which occurs in an electric conductor having a varying current applied thereto. Thus, in such a conductor, the electric current density is greatest at the surface.
Moving the object closer to the coil increases the effect. Thus, coil characteristics are a function of the spacing between the coil and the object and the eddy current effect has been relied upon, to a great extent, in the development of proximity sensors, contactless tachometers, thickness measuring devices, position detecting systems, accelerometers and monitoring devices of various types.
For instance, utilization of the eddy current effect has been employed in many forms of sensors relied upon to determine the position of a specific object or target, as in the case of proximity sensors. In such systems, an electronic circuit is employed to generate an analog signal related to a target's position. In some systems, the sensing coil is made part of an impedance bridge circuit so that the changes in coil inductance and impedance produce error signals related to target position. In other systems, the coil is part of an oscillator circuit so that the target's movements produce frequency changes. The frequency of excitation of the coil is frequently 1 MHz or more, and at these frequencies eddy currents are generally confined near the surface of the target due to the skin effect. For example, the skin effect depth in aluminum at 1 MHz is 0.0033 inch. While targets for such detectors may be very thin and light-weight, thicknesses which produce maximum eddy currents at individual operating frequencies are obviously selected. In such systems where sensing occurs as a function of the output of the driving oscillator circuit, a sensing condition invariably results in a frequency shift at the oscillator since the resulting eddy currents induced in the target which create the sensed condition work a change in the effective inductance of the coil, and hence, a resulting frequency change in the oscillator circuit in a driving relationship therewith.
Another form of proximity transducer could be devised using the principles of mutual resonance. For example, a sensing coil which forms part of an oscillating circuit could use as its target a passive resonant circuit tuned to the same frequency as that of the oscillator. The passive resonant circuit would absorb energy from the oscillator coil in proportion to its proximity. Under these conditions, a sensed condition would be characterized by a reduction in amplitude in the output of the driving oscillator rather than the frequency changes which attended the creation of substantial eddy currents and the attendant inductance change in the coil. Here the passive resonant circuit is effectively stealing energy from the driving circuit rather than working a change of impedance in the linking coil. If the passive resonant circuit is positioned closely to the sensing coil, the oscillation of the driving circuit may cease entirely.
While use of a passive resonant circuit has the advantage that it offers far more sensitivity than eddy current systems, it has disadvantages. These disadvantages include a requirement that the target consist of a coil and capacitor combination which is cumbersome. In addition, the stability requirements for the oscillator frequency are stringent and the effects of the environment on the target can substantially impact its tuning, and hence, its response. For these reasons, eddy current systems today have found wider application than those associated with passive resonant circuits.
Another use of eddy current systems is in electronic coin detectors such as are disclosed in my U.S. Pat. Nos. 4,354,587 and 4,359,148. Here, the loss associated with a valid coin passing through a coil oscillating at a frequency within the RF range is relied upon to create a well-defined notch in the output of the oscillator as a coin traverses a coil. This notch is so well defined that other coins or slugs having differing physical characteristics can readily be distinguished and determined to be unacceptable when viewed from the standpoint of the creation of a smaller or greater notch than that associated with a valid coin. It was through experimentation with this form of coin acceptor that the instant phenomena was discovered.
More particularly, while conducting experiments to determine the susceptibility of such electronic coin detectors to slugging, a phenomena was noted which subsequently led to the discovery that thin layers of non-magnetic conductive material appear to act in the same manner as a passive resonant circuit.
In the experiments conducted, a version of a slug wherein a thin metallic layer was placed on a plastic washer was employed. In fact, the thin metallic layer was the paper-backed foil employed in packaging Marlboro cigarettes. This foil was effectively Scotch-taped to the plastic washer in such a manner that only a single layer was placed thereon. The effect of this form of slug was surprising and wholly unexplainable in that insertion of this slug created the largest loss in the output of the driving oscillator that had been experienced.
This large loss was produced by an extremely thin piece of aluminum foil which behaved completely differently from any of the metallic coins or metallic slugs undergoing test because until then the loss produced had been generally proportional to the mass of the coin or slug being tested. Another disparity was noted in that the output of the oscillator did not indicate the presence of an eddy current mechanism within coins or slugs being tested. More particularly, depending upon the mass and other characteristics of the coin or slug being tested, the output of the oscillator driving the coin detector normally acts as if one or more turns on the coil are shorted. The reduction in the output of the oscillator is attended by marked changes in frequency as the circuit is detuned. Here, however, not only was the attenuation in the output of the oscillator larger than anything previously experienced, but it was not attended by any shift in the frequency indicative of an increase in the resistance reflected at the coil. However, since the foil-clad slugs were not treated as authentic coins by the electronic coin detector, the results were not further investigated at this time, but instead, the effects observed were noted for further investigation at another time.
Further inquiry into the unusual phenomena noted during the slugging experiments confirmed the fact that extremely thin pieces of aluminum behaved completely different from any metallic coins tested within the coil of electronic coin detectors, and instead acted in the same manner as a passive resonant circuit which was self-calibrating with respect to frequency. It was also found that any non-magnetic conductive material such as sheet brass, aluminum, copper, gold, silver, zinc, or almost any other form of conductive material acted in a similar manner to remove energy from the coil associated with the tuned oscillator driving the electronic coin detector. The initial observation that this did not appear to be an eddy current effect has been confirmed by further experimentation.
The further experimentation conducted indicated that the effect noted and being analyzed was characterized by behavior corresponding to the effect of placing a passive resonant circuit close to the coil of an active resonant circuit where both circuits were tuned to essentially the same frequency. However, it was also noted that frequency dependence was not highly critical in that the thin foil under test would tend to calibrate itself to the frequency of the oscillator even under circumstances where the tuned frequency was deliberately and substantially shifted. Furthermore, it was found that the resonant characteristics of the thin film of material was distance dependent to a large degree and the same could be maximized by configuring the coil or the film to a shape enabling a uniform distance between all portions of the coil and the film to be achieved. This was done, as shall be seen below, by developing flat, helically disposed coils on printed circuit boards, or conversely, by configuring the film to conform to the cross-section of a coil by designing the same as a piston or the like. When this was done, the correspondence of the effect noted to that of a passive resonant circuit became manifest. The phenomena observed has been termed spontaneous resonance.
While the phenomena herein referred to as spontaneous resonance is not fully understood, experimentation has confirmed its existence in all forms of thin conductive media and a clear interrelationship between the thickness of the material employed and the frequency of the oscillator. This indicates clearly that the skin effect depth of the material is involved in that thicknesses for a film selected must be in the range of a fraction of its skin effect depth. To date, a full range of parameters in terms of frequency versus thickness are not available for a substantial number of materials since some difficulty has been experienced in obtaining thin layers of varying materials and varying thicknesses in the ranges involved. However, for a number of materials, frequencies and thicknesses, it has been confirmed that the effect termed spontaneous resonance does exhibit marked correspondence to that manifested by a passive resonant circuit tuned to the frequency of oscillation of the driving circuit. Furthermore, experimentation conducted to date indicates that for a given frequency, the effect of spontaneous resonance is only present for a predetermined range of thickness. If a layer of material exceeds this thickness range, the material acts inductive, producing eddy currents; while layers which are thinner than the range involved act capacitive. Thus, for a frequency of 6.9 MHz, a thin layer of aluminum begins to appear inductive at approximately 12 microns and capacitive at 1 micron.
When such thin layers are placed outside a certain range, no effect on the oscillator is noted. Within a predetermined range, the output of the oscillator is reduced in what appears to be an increasing fashion as the film is brought closer to the coil associated with the tuned driving circuit. Within a certain distance the presence of the thin layer will completely shut down the oscillator. Furthermore, experimentation has confirmed that this effect is only present when the oscillator coil effectively sees a thin layer, and hence, presence of a thicker metallic layer, such as a coin, in the vicinity of the film will completely destroy the spontaneous resonance exhibited thereby. These outstanding and wholly unexpected characteristics associated with the phenomena has enabled self-calibrating, contactless sensors, switches and modulators to be developed in accordance with the teachings of the instant invention.
Therefore, it is a principal object of the present invention to provide self-calibrating, contactless sensors, switches and modulators employing the apparent resonant properties of thin, non-magnetic conductive materials.